SHERLINE MACHINIST'S
TIPS

TIP

20--Making
and using broaches to make non-round
holes/Rich Dean

Broaches are used when
you need to make holes that are shapes
other than round. A round pilot hole is
first drilled, and then a broach of the
desired shape is pressed through the
hole to remove additional metal, leaving
the desired shape. Square, hex,
"D" and other shaped holes can
be achieved in this manner. Broaches
with multiple steps like the ones shown
below take less force to use because
only a portion of the shape is removed
by each step as the broach goes through
the hole. The first step is the size of
the pilot hole and the last step is the
size of the desired finished hole.

The broaches shown above are
plain lathe turned, propane torch
hardened and tempered in oil. You will
have to decide the overall size, shank
and dimensions to use for your
particular project. Use W1, O1 or
whatever drill rod (tool steel). No
reason to be fussy.

MAKING
THE BROACH--Turn or machine
the profile required to full dimensions,
full length. On one end, turn a pilot
tip to fit the hole to be broached. If a
full form cut is required, this will be
the minimum diameter. Otherwise, make
the hole oversize to broach only the
corners out. That makes it easy on the
broach. Next, cut chip slots which form
the cutting edges a little smaller in
diameter than the pilot diameter. The
more edges you have, the less the chip
load as the broach is cutting. Now set
the lathe topslide (compound slide) to
about a 3-5 degree angle to form the
clearance angle behind each edge. The
amount taken off of each edge is
greatest at the pilot end and gets
progressively less as you go toward the
shank end. Divide 1/2 the diameter
difference by the number of edges (minus
1) to find the amount of clearance to
cut per edge. Leave the last edge for a
full cut.

HARDENING--Using
a propane torch, heat the working end to
a cherry red in dim light and then dunk
the heated part in oil. (An open gallon
of motor oil is fine.) Polish a few
spots to reveal clean metal and reheat
slowly until you see the bright steel
areas turn to a light straw color.

USING THE BROACH--To use, support the stock being
broached close up to the hole, drop in
the broach and push thru with an arbor
press. Do Not hammer or jam thru by
hand. Any misalignment will snap it off.
Always, even with brass, lube generously
with oil. Also, on full cutting
broaches, chip accumulation may jam it,
so extract, clean out and push thru for
the final cut.

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